The requirements for positive brake locking devices for air braked trailers are stringent. Trailers left with brakes locked under the control of air pressure in storage tanks on the trailer are subject to unintended release from leakage which results in the loss of pressure in the storage tank, thereby leaving the trailer free to roll.
Heretofore, brake locking devices have been provided of various types. Representative of these types are described in U.S. Pat. Nos. 2,041,065; 3,759,147; and 3,874,747. These prior devices were specifically designed for use with trailers where only one storage tank was required. To achieve a mechanical lock, a spring was used to push a pawl downward to connect with a ratchet on the brake rod of the brake actuator chamber. The pawl was forced downward against the remaining force of the trailer's air supply, making it difficult to consistently obtain a mechanical lock on the brakes. Therefore, under certain road conditions, trailers utilizing such prior devices were vulnerable to roll off.
Whenever there is a failure in any of the components designed to contain compressed air of a brake system, such as a leak in a brake delivery hose, the air tank itself, diaphragms or pneumatic valves, it becomes impossible to release the brakes with the trailer's energy supply. Heretofore, the only way to release the brakes was to manually make the brake inoperative. In such cases, the vehicle could become a hazard and become dangerous.
Accordingly, there is a need for an improved control unit for a parking brake system for tractor-trailer vehicles where two tanks of pressurized air are required in order to ensure that the brakes can be released in the event of a failure in the compressed air delivery system.